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Martin Greven: Unconventional Magnetism in Copper-Oxide Superconductors


The study of correlated-electron materials offers myriad opportunities to discover and investigate novel fundamental magnetic, structural, and electronic phenomena and phases. A muchstudied example is the so-called pseudogap regime in the normal state of the high-Tc cuprates, in which these materials exhibit highly unusual electronic and magnetic properties. Neutron scattering experiments play an invaluable role in this endeavor, since they provide essential structural and magnetic information about new phases of matter and the transitions between them. In fact, neutron scattering is the only experimental probe that allows the measurement of the entire momentum- and energy-dependent magnetic susceptibility of a condensed matter system. The importance of this experimental technique has been recognized throughout the world and motivated the upgrades of facilities and the construction of new ones, including the Spallation Neutron Source at Oak Ridge National Laboratory. Advanced crystal growth is of enormous importance to the field of correlated-electron materials. The crystal-size requirements for cutting-edge magnetic neutron scattering experiments are particularly demanding, and often can be met only through a very focused growth effort.

In a major breakthrough, our group developed a method to grow sizable, high-quality single crystals of the mercury-based compound HgBa2CuO4+δ (Hg1201), the first member of the family of materials with the highest known values of Tc. This has given us the unique opportunity to begin to explore in detail the arguably most desirable family of cuprate superconductors. In particular, it has enabled recent polarized neutron scattering measurements that revealed a novel magnetic order and associated magnetic excitations in the pseudogap regime. The order gives rise to Bragg peaks at the two-dimensional Brillouin-zone center (q2D=0), and hence breaks time-reversal symmetry, but not the translational invariance of the crystal lattice. A highly unconventional proposal for the origin of this hitherto hidden magnetism is that it arises from circulating charge currents within the unit cell that may involve the apical oxygen orbitals.

Schematic phase diagram of the copper-oxide superconductors

Schematic phase diagram of the copper-oxide superconductors.

Novel magnetic excitation recently discovered below the pseudogap temperature (T* ~ 200 K for the present sample) in the hole-doped compound HgBa2CuO4+δ using polarized magnetic neutron scattering. The excitation exhibits a weak dispersion from the two-dimensional zone center to the zone corner.

Novel magnetic excitation recently discovered below the pseudogap temperature (T* ~ 200 K for the present sample) in the hole-doped compound HgBa2CuO4+δ using polarized magnetic neutron scattering. The excitation exhibits a weak dispersion from the two-dimensional zone center to the zone corner.

In order to arrive at a deeper understanding of the high-Tc cuprates and to elucidate the mechanism of superconductivity, we propose to extend the study to other hole-doped compounds, and in particular to (La,Sr)2CuO4 (LSCO), and to clarify the similarities and differences between the hole- and electron-doped sides of the phase diagram.

In particular, will search for the novel magnetic excitations in hole-doped LSCO. Like Hg1201, LSCO is a single-layer compound, but with a much lower value of Tc, probably due to the stronger influence of disorder. LSCO has a body-centered tetragonal structure at high temperature and becomes orthorhombic at low temperatures, which differs from the simple tetragonal structure of Hg1201. The dimensions of the CuO6 octahedron in LSCO are also quite different – the distance between apical oxygen and the copper-oxygen plane is 2.4 Å, compared to 2.8 Å for Hg1201. Therefore, extending our study to LSCO will be particularly revealing, since the novel magnetism likely involves the full CuO6 octahedron. One advantage of studying LSCO is the easy access to essentially the full doping range, including the heavily underdoped (low hole concentration) part of the phase diagram. Another advantage is that large single crystals are relatively easy to grow, and our group has the required expertise and equipment.

We will also search for the new magnetism in electron-doped Nd2-xCexCuO4+δ (NCCO). Neutron scattering is a powerful probe of magnetism, and for the study of low-dimensional systems with strong quantum fluctuations, it requires the use of large single crystals. Past difficulties with the growth and characterization of the electron-doped cuprates have led most researchers to focus on the hole-doped side of the phase diagram. We have partially overcome this problem and have learned to grow sizable NCCO crystals using the traveling-solvent floating-zone (TSFZ) imagefurnace technique. Importantly, we have found that even though the best crystals contain secondary phases and disorder, it is possible to extract fundamental new insight from magnetic neutron scattering experiments. The study of NCCO will help answer the pivotal question whether q2D=0 magnetism is universally associated with the pseudogap phenomenon, regardless of the type of the charge carriers (electrons or holes) and of the presence of apical oxygen atoms, which are absent in electron-doped systems.





Funded by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-1420013


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UMN MRSEC

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